In recent years, studies are being aggressively conducted to develop a white light-emitting diode with the light-emitting source being a blue light-emitting element using a nitride-based compound semiconductor (InxAlyGa1-x-yN, wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1). The white light-emitting diode is lightweight, does not use mercury and has long life, and therefore, demands are expected to expand in the future. A most commonly employed method for converting blue light of a blue light-emitting diode into white light is a method where, as described, for example, in Japanese Unexamined Patent Publication (Kokai) No. 2000-208815, a coating layer containing a fluorescent material capable of absorbing a part of blue light and emitting yellow light and a mold layer for mixing blue light of the light source and yellow light from the coating layer are provided on the front surface of a light-emitting element which emits blue light and a pseudo-white color is obtained by mixing the blue color and the yellow color which are in a complementary colors relationship with each other. As for the coating layer, a mixture of a cerium-activated YAG (Y3Al5O12:Ce) powder and an epoxy resin has been conventionally employed. However, in this method, for example, uneven distribution of the fluorescent powder contained or a fluctuation in the amount of fluorescent powder among individual light-emitting diodes readily occurs when applying the coating layer, and color unevenness is a problem.
In order to avoid this problem, Japanese Unexamined Patent Publication (Kokai) No. 2003-204080 has proposed a method of forming a nitride semiconductor layer comprising InxAlyGa1-x-yN (wherein 0≦x≦1, 0≦y≦1 and 0≦x+y≦1) on a substrate of which main plane is the (111) plane of Y3Al5O12:Ce fluorescent material single crystal, causing blue light emitted from the light-emitting layer to be incident directly on the substrate, and emitting homogeneous yellow fluorescence from the substrate itself, whereby a homogeneous white color free of color unevenness is obtained only by a light-emitting chip without using a fluorescent powder-containing coating layer (see, FIG. 2).
However, in the method described in Japanese Unexamined Patent Publication (Kokai) No. 2003-204080, the difference in the lattice spacing between the Y3Al5O12 (111) substrate and InxGa1-xN of the nitride semiconductor buffer layer formed thereon is still a value larger than the difference in the lattice spacing between the Al2O3 single crystal (0001) plane (hexagonal notation, hereinafter the same) substrate in the method commonly employed at present and, for example, a GaN buffer layer. Therefore, a larger number of distortions are generated in the nitride semiconductor layer of this method than in the layer formed by the method employed at present, and a good-quality nitride semiconductor light-emitting layer and in turn, a good light-emitting diode is difficult to obtain.
CrystalLattice SpacingInxGa1−xN3.19 Å to 3.53 ÅY3Al5O128.5 Å (on (111) plane)GaN3.19 ÅAl2O32.74 Å (on (0001) plane)
Considering the method commonly employed at present for producing a nitride semiconductor layer, use of the Al2O3 single crystal (0001) plane having a narrower lattice spacing than the YAG single crystal (111) plane is apparently preferred in order to obtain a good-quality nitride semiconductor layer. However, when an Al2O3 single crystal is used, a yellow fluorescence cannot be obtained and the above-described coating layer becomes necessary, giving rise to a problem such as color unevenness.
Also, in the single crystal Y3Al5O12:Ce fluorescent material substrate used for the method of Japanese Unexamined Patent Publication (Kokai) No. 2003-204080, the inside is homogeneous to allow light to go straight but not be refracted, reflected or scattered and therefore, as described in Jpn. J. Appl. Phys., Vol. 41, pp. L887-L888 (2002), light entered has a high probability of repeating total reflection at the interface (hereinafter referred to as an “inside surface”) between the substrate and the outer material (e.g., air, resin) and attenuating without going outside, as a result, good light out put efficiency cannot be obtained.
It could therefore be advantageous to provide a light-emitting diode substrate for forming a semiconductor, ensuring that the crystal-structure matching a semiconductor for the formation of a light-emitting diode is good, a good semiconductor layer with less defects can be formed, good-efficiency light emission can be obtained from a light-emitting layer formed in the semiconductor layer, uniform florescence can be emitted by light from the light-emitting layer in the semiconductor layer, and the light can be efficiently out put. It could also be advantageous to provide a color unevenness-free light-emitting diode using the substrate.